(a) Field of the invention
The present invention is related to pickup cartridges for use in record playback system to reproduce signals recorded on a record disk, and more particularly to stereophonic pickup cartridges of moving-coil type and moving-magnet type.
(b) Description of the prior art
In a 45--45 stereophonic record disk, as well known, left- and right-channel signals are separately recorded or carved on the surfaces of the individual walls which constitute a groove of a record disk. These wall surfaces are inclined symmetrically at an angle of 45 degrees relative to the surface of the disk, and face each other at right angle. The recorded signal waveshapes carried on these wall surfaces can be reproduced in a pickup cartridge by first detecting these signal waveshapes by means of a vibration system, which may include a stylus tip secured on a cantilever for tracing the record groove, and an armature which vibrates jointly with the cantilever within a magnetic field, and then by converting the movements of the vibration system to electrical signals by means of a displacement-electromagnetism conversion system including the armature and coils.
In a conventional known stereophonic pickup cartridges of the moving-coil type and the moving-magnet type, the displacement-electromagnetism conversion system is designed so that the component of movement of the vibration system in a direction perpendicular to one of the wall surfaces of the record groove is directly converted to an electrical signal to provide a reproduced left-channel signal, and that the component of movement of the vibration system in a direction perpendicular to the other one of the wall surfaces of the record groove is directly converted to an electrical signal to provide a reproduced right-channel signal. In a pickup cartridge having a conversion system of the aforesaid arrangement the reproduction of signals with good channel separation requires a high balance between right- and left-channels in such respects as vibration characteristics of the vibration system and conversion efficiency of the movement-electromagnetism conversion system.
On the other hand, the movement of the vibration system in a pickup cartridge tends to be placed under an influence of several factors such as (a) extraneous vibration, (b) the weight of the cartridge and (c) unnecessary vibration at very low frequency due to warping of a record disk used. The extent of influence of these factors upon the movement of the vibration system is dependent on the directions of components of movement of the vibration system. Namely, the weight of the cartridge will affect mainly the component of movement of the vibration system in vertical direction. Vibrations due to warping of the record disk also contribute to the above-said influence upon the component of movement of the vibration system in a vertical direction. For the foregoing reasons, the vibration system is required to possess a vibration characteristic for its vertical component of movement which is different from that for horizontal component of movement. For example, it might be preferable to set larger the stiffness of the vibration system which acts only for the vertical component of movement of this system than for the horizontal component of movement for the purpose of decreasing such ill influence due to warping of the record disk. According to the aforementioned arrangement of the known pickup cartridge, however, it has been very hard to satisfy the requirement for imparting different vibration characteristics for vertical and horizontal movements without the accompanying of degradation of channel separation and other electrical output characteristics. This problem of satisfying the above-said requirement without degradation of channel separation might be considered to be overcome by increasing the stiffness for the horizontal component of movement in accordance with an increase in stiffness for the vertical component of movement and vice versa. If so arranged, however, the sensitivity of the cartridge for reproducing signals of a low frequency range will become decreased, leading to a poor frequency response characteristic, since the low frequency components of the recorded signals are picked up mainly by following the horizontal component of movement of the vibration system.
So far as a pickup cartridge of the variable-reluctance type is concerned, its movement-electromagnetism conversion system is designed so that the component of movement of the vibration system in each of the vertical and horizontal directions is individually detected. Therefore, it may be possible to impart such vibration characteristics of the vibration system as are different between vertical and horizontal movements thereof. However, this known pickup cartridge of the variable-reluctance type has disadvantages including relatively complicated arrangement, poor stability of operation, and large effective mass of the vibration system measured at the stylus tip. Further, it is disadvantageous in that the arrangement for detecting the vertical movement is much different from that for detecting the horizontal movement, which will be an obstacle to attain a high channel separation characteristic.
Further discussion of a known pickup cartridge of the moving-coil type will be made hereunder. In almost all of the ones of this type of cartridge, the moving coils are designed to make circular movements, cutting magnetic fields obliquely, and a relatively large number of parts such as coils per se, a supporting member for these coils and a damping member for damping the vibrations of the vibration system are located in a relatively wide air gap where a magnetic field is established. Thus, it is hard to produce a sufficient intensity of the electromotive force in the coils. Accordingly, in such known pickup cartridge of the moving coil type, the possible efficiency of movement-electromagnetism conversion is limited to a relatively low level.